Various factors have contributed to move hybrid vehicles from a niche market closer to the mainstream. While the early impetus for hybrid vehicles (or other “green” vehicles) was a reduction in the pollution generated thereby, the upward trend in oil prices has added an economic appeal to hybrid vehicles for some buyers. Additionally, improvements in hybrid technology have resulted in hybrid vehicles that, while still offering somewhat enhanced fuel efficiency, are marketed for enhanced performance relative to conventional vehicles.
However, hybrid vehicles remain prohibitively expensive for many prospective vehicle buyers, and particularly those hybrid vehicles which approximate the size and comfort of a conventional vehicle. Additionally, the efficiency gains of current commercially available hybrid vehicles are relatively modest in comparison to small conventional vehicles with high efficiency motors.
Electric-only vehicles are also appearing more prominently on the market, although the usefulness of such vehicles to many drivers as substitutes for conventional vehicles (unlike hybrid vehicles) remains limited by battery capacity. Batteries, in general, are an issue for electric-only and hybrid vehicles, as battery costs can be much higher than for conventional batteries. The high cost of batteries is aggravated by the need for multiple battery replacements during the anticipated life of many such vehicles.
Pneumatic motors and compressors have also been used with some success in vehicles. However, these systems, while promising in their own respect, have been the subject of only very limited incorporation with other vehicle energy systems.